Well, I brewed some beer this weekend and upon further review of what I did it sure seems that I didn't use enough bittering hops (it calculates out to have a bitterness of around 20).

I'm not what you would call highly experienced at this (my second batch) but I'm curious if this is going to be too sweet to drink, and then if so is there a way to increase the bitterness now that it's fermenting?

You should be fine. Most holiday spiced beers that I've tried had around 20-30 IBU's. Your focus in this beer is more on the spices. If you're worried it's going to be too subtle, you might be able to dry hop it when you rack to secondary. I don't know how well that would work with the style you're aiming for though.

There is really no detectable spicy flavor to it, although both my wife and I agreed it was fairly tasty.

My issue now is that the fermentation (which started out fairly weak) only got about 1/2 done before stopping. I stopped at my local store and they suggested adding some more yeast as we ruled out any other problems while I was there.